This invention relates to fuel cells and, in particular, to a high temperature fuel cell assembly which employs internal reforming of the supply fuel in such a way as to improve the temperature distribution within the fuel cell assembly.
It is customary in the use of fuel cells to arrange the cells in an assembly, usually a stack, to produce useful power levels. It is also customary to utilize a hydrogen containing fuel such as methane as the fuel supply for the fuel cell assembly and to reform this fuel to produce hydrogen containing fuel process gas for flow through the assembly.
In a high temperature internally reforming fuel cell assembly, such as, for example, an internally reforming molten carbonate fuel cell assembly, the fuel supply is reformed internally of the assembly to produce the fuel process gas. This gas is then conveyed through the anode compartments of the fuel cells of the assembly. As it passes through the anode compartments, the fuel process gas undergoes an electrochemical reaction with an oxidant process gas which is carried in the cathode compartments of the fuel cells of the assembly. The electrochemical reaction occurs via an electrolyte which separates the anode and cathode compartments and which conducts electrically charged ions therebetween. This results in the desired production of electrical energy or output from the assembly.
An internal reforming high temperature fuel cell assembly is advantageous in that it avoids the need for expensive and complex external reforming equipment. In addition, the reforming reaction, which is endothermic (i.e., absorbs or requires heat) can be used advantageously to help cool the assembly.
The reforming of the fuel supply in an internally reforming fuel cell assembly is realized by using a steam reforming catalyst. The catalyst is placed within the assembly in the path of the hydrocarbon fuel supply, to thereby reform the fuel supply and produce fuel process gas. Two forms of internal reforming have been used, direct and indirect.
Each is based on the placement of the reforming catalyst in a particular relationship with respect to the anode compartments of the fuel cells of the assembly.
In direct internal reforming, the reforming catalyst is placed in anode passages of the fuel cell anode compartments which directly carry hydrocarbon fuel to the anode electrodes of the fuel cells, i.e., in anode passages which directly communicate with the anode electrodes. This has the advantage of directly providing the hydrogen-containing fuel gas resulting from the reforming process to such electrodes. However, in this type of arrangement, since the reforming catalyst is in the anode passages which directly carry hydrocarbon fuel, the catalyst is exposed via the anode electrodes and anode passages to the electrolyte of the assembly fuel cells. This exposure over time degrades the catalyst performance.
In indirect internal reforming, the reforming catalyst is placed in chambers or passages within the fuel cell assembly which are isolated or removed from the anode passages which directly communicate with the anode electrodes. The reformed process gas is then routed to these anode passages for electrochemical reaction. The advantage of indirect internal reforming is that since the reforming catalyst is situated in the isolated passages, the catalyst is better protected from poisoning or degradation by the fuel cell electrolyte.
U.S. Pat. No. 4,182,795 describes a system and method in which a high temperature fuel cell employs indirect internal reforming via passages isolated from the anode passages directly carrying the fuel supply gas to the anode electrodes. In this system and method, the flow in the isolated passages is set independently of that in the anode passages based on the overall quantity of cooling desired. Also, separate ducting for the two flow paths and external junctions and valves are used to deliver the reformed gas to the anode electrodes.
U.S. Pat. No. 4,365,007 discloses a fuel cell system and method employing direct internal reforming. In this case, the reforming catalyst is placed in a passage which communicates through a porous barrier with the anode passages directly carrying fuel supply gas to the fuel cell anodes. The porous barrier acts to partially isolate the catalyst from the electrolyte of the fuel cells, and the system further relies on a pressure difference between the catalyst containing passages and the anode passages to provide reformed gas to the anode electrodes and to prevent electrolyte vapor from reaching the catalyst. The costs of this system are high due to the need for a complex anode current collector to provide the isolated and anode passages as well as the extra material of the porous barrier. Also, the uniform delivery of reformed fuel gas to the anode passages through the porous barrier using a differential pressure may be difficult to realize.
U.S. Pat. No. 4,567,117 discloses a technique which can be used for a fuel cell using either indirect or direct internal reforming. In this case, the catalyst employed for reforming is tailored to promote uniform temperature distribution in the fuel cell. In particular, the catalyst is applied directly to those parts of an anode current collector which form either isolated passages or anode passages and is actively distributed so as to reform more gas in hotter areas of the fuel cell thereby reducing temperature non-uniformity. The limitations for direct internal reforming and indirect internal reforming discussed above for the '795 patent and the '007 patent apply here as well depending upon whether the technique of the patent is applied in a direct or an indirect internally reforming fuel cell.
U.S. Pat. No. 4,788,110 describes a technique usable in a direct internal reforming fuel cell in which the anode current collector forms catalyst containing passages which are partially shielded from the anode passages which are also formed by the current collector. With this configuration, the partial shielding of the catalyst makes it less able to participate in the reforming process and thereby reduces its effectiveness.
U.S. Pat. No. 5,175,062 describes an indirect internal reforming fuel cell stack with reforming units disposed at intervals along the stack length. The reforming units each contain a U-shaped catalyst containing chamber or passage having a fuel feed port at its corner. The reformed gas from the reforming units is passed to a manifold which feeds the anode passages communicating with the anode electrodes of the stack fuel cells for electrochemical reaction. In this system, due to the required size of the fuel feed tube, a relatively high fuel gas pressure drop is experienced and the cost of the system is increased.
U.S. Pat. No. 5,348,814 describes an indirect internal reforming fuel cell stack also with reforming units distributed along the stack length. In this stack, the manifolding is internal. Due to the complexity of the bipolar plate used in the stack to form each reforming unit, the cost of the stack is relatively high.
U.S. Pat. No. 5,660,941 discloses an indirect internal reforming fuel cell stack with various configurations for a catalyst member to be used in the isolated reforming chambers. A plate-like catalyst member and a mesh type member supporting catalyst pellets for insertion over peak regions of an anode current collector are described.
U.S. Pat. No. 4,877,693 describes a fuel cell stack employing both indirect and direct internal reforming. The indirect internally reforming is carried out by using catalyst-containing passages distributed along the fuel cell stack length. These passages are isolated from the anode passages directly carrying fuel supply gas to the fuel cell anode electrodes of the stack. The direct reforming is carried out via catalyst placed in the anode passages. In this case, the fresh supply gas is passed through the catalyst-containing passages and is partially reformed. The partially reformed gas is then passed through the anode passages where it is further reformed and where the reformed gas undergoes electrochemical reaction. A manifold is used to couple the partially reformed gas from the isolated catalyst-containing passages to the anode passages.
A more recent fuel cell stack that employs both indirect and direct internally reforming is disclosed in U.S. patent application Ser. No. 10/269,481, assigned to the same assignee hereof. In this stack, the catalyst-containing isolated passages are in the form of reforming units which are distributed along the length of the stack and which contain U-shaped flow paths. The outlets of the reforming units and the inlets of the anode compartments with the catalyst-containing anode passages communicate with a common manifold so that the partially reformed supply gas from the reforming units is passed to the anode passages. In this system also, the fuel supply feed is placed in the common manifold to mitigate against system leaks.
It is an object of the present invention to provide a fuel cell assembly having both direct and indirect internal reforming and which is better able to adapt to changes in the fuel cell assembly over the life of the assembly;
It is a further object of the present invention to provide a fuel cell assembly having both direct and indirect internal reforming and which is capable of better realizing desired temperature distribution and performance when fuel of different composition is used; and
It is yet another object of the present invention to provide a fuel cell assembly having both direct and indirect internal reforming and which is capable of better realizing desired temperature distribution and performance in the face of catalyst degradation.